In an automated overhead monorail system, for example, of the type used in an automotive assembly plant, individually controlled trolleys or carriers are each driven by a drive unit which includes an electric motor and gearbox assembly having an output shaft. A friction drive wheel is mounted on the output shaft and engages the top of the monorail, and the gearbox may incorporate a clutch which is actuated by a forwardly projecting control shaft supporting a laterally projecting shift lever or arm. The arm engages projections or ramps on the monorail, and pivoting of the arm is effective to operate the clutch within the gearbox or actuate a switch which controls the electric motor.
In such a carrier drive unit, it is desirable to provide for positive engagement and disengagement of the clutch while the motor is running and while the drive unit is moving a substantial load supported by the carrier. It is also desirable for the clutch mechanism to provide for overtravel of the outwardly projecting shift arm in order to accommodate variations in the locations of the lever actuating ramps along the monorail. It is also desirable for the unit to incorporate an electrically actuated brake which quickly stops the drive motor shaft and also to provide for manually moving the carrier with little force when the clutch is disengaged. In addition, it is desirable for the gearbox to be driven by a standard electric motor and to provide for an extended and dependable service life with minimum down time for maintenance.